1. Field of the Invention
The present invention relates to phase-change optical recording media recordable and reproducible over a broad range of linear velocities.
2. Description of the Related Art
An example of phase-change optical recording media having high compatibility in reproduction by DVD-ROM drives can be found in Japanese Patent Application Laid-Open (JP-A) No. 2002-237096. However, this optical recording medium does not take high-velocity recording and good downward compatibility (compatibility in recording at low linear velocity) into consideration.
Examples of means for achieving satisfactory recording at high velocity can be found in Japanese Patent (JP-B) No. 3150267, JP-A Nos. 2001-56958 and 2002-96560, in which the composition of a recording layer is specified to one suitable for high-velocity recording; in JP-A Nos. 2002-237095, 06-195747, 11-339314, 2001-167475 and 2002-222543, in which a layer mainly aiming to accelerate crystallization is arranged adjacent to a recording layer; and in JP-A Nos. 2002-100075 and 2002-237088, in which a reflective layer comprising Ag or an Ag alloy is provided. These optical recording media are intended to achieve high-velocity recording by increasing the crystallization rate of the recording layer. However, these techniques alone cannot provide optical recording media capable of high-velocity recording and having good downward compatibility as in the present invention.
JP-A No. 2002-190138 discloses an optical recording medium having two upper protective layers. This optical recording medium may have good overwrite properties over a broad range of linear velocities but does not take downward compatibility into consideration.
JP-A No. 2002-245663 discloses an optical recording medium recordable at linear velocities ranging from 1.2 to 30 m/s. This optical recording medium is recordable at high velocity but does not take downstream compatibility into consideration.
DVD+RW medium is a kind of phase-change optical recording media, on which recording can be performed repeatedly, and which have high compatibility with DVD-ROM. They are specified in “DVD+RW 4.7 G bytes Basic Format Specifications System Description” and are practically used as recording media for dynamic images or external archival media for personal computers.
In a phase-change optical recording medium, a thin film of recording layer on a substrate is heated by the irradiation with a laser light, the recording layer thereby undergoes phase change between a crystalline phase and an amorphous phase. Thus, the reflectance of the medium is changed to thereby record and erase information on the medium. In general, the recording layer before recording is configured to have a crystalline phase having a high reflectance. A recording is produced by forming a mark of an amorphous phase having a low reflectance and a space of a crystalline phase having a high reflectance. A method of repeated recording employed in DVD+RW is shown in FIGS. 1 and 2.
In this method, a recording pulse pattern is used in which a laser power is modulated at three power levels of a peak power Pp, an erasing power Pe, and a bias power Pb (Pp>Pe>Pb). Upon irradiation of a laser light which is pulsed with a pulse train consisting of Pp and Pb, the recording layer undergoes fusing and quenching multiple times to thereby form an amorphous mark. Upon irradiation with Pe, the recording layer is gradually cooled to thereby form a space.
In the case of recording on DVD+RW media, four parameters, dTtop, Ttop, Tmp, and dTera shown in FIG. 1 specify a recording pulse pattern. Herein T is a reference clock; a pulse train between the front and rear of the pulse train consisting of Pp and Pb is referred to as a “multipulse” and is arranged with a cycle of 1T; dTtop is a front pulse starting time, Ttop is a width of the front pulse; Tmp is a width of a peak power pulse in the multipulse; Tw is a width of the reference clock; and dTera is an erase-starting time.
DVD+RW media is recordable and reproducible at 1× to 2.4× speed (3.49 to 8.44 m/s). To achieve higher capacity data archival, demands have been made on media that is reproducible at higher velocity. As described in the aforementioned patent documents, media which is recordable at high velocity are generally obtained by specifying the composition of a recording layer or by providing a layer for accelerating crystallization adjacent to the recording layer to thereby increase the crystallization rate of the recording layer. However, when the crystallization rate of the recording layer is increased, a record is not satisfactorily produced unless a higher recording power is used or a recording pulse wave pattern out of the multipulse of 1T period is used. Accordingly, when a drive recordable at 1× to 2.4× speed with a maximum recording power of 15 mW is used on these media, records cannot be produced thereon due to insufficient power. In addition, firmware alone cannot significantly change the recording pulse pattern.
Generally, a phase-change optical medium comprises a transparent plastic substrate, a specific groove formed on the substrate, and a thin film arranged thereon. A polycarbonate is generally used as a material for the plastic substrate, and the groove is often formed by injection molding. The thin film formed on the substrate is a multilayer film basically comprising a lower protective layer, a recording layer, an upper protective layer, and a reflective layer formed on the substrate in this order. The lower and upper protective layers comprise, for example, an oxide, a nitride, or a sulfide, of which a mixture of ZnS and SiO2 is often used. The recording layer often comprises a phase-change material mainly comprising SbTe. Such phase-change materials include, for example, Ge—Sb—Te, In—Sb—Te, Ag—In—Sb—Te, Ge—In—Sb—Te, Ge—Sn—Sb—Te, and the like. The reflective layer generally comprises a metallic material, of which Al, Ag, Au, Cu, other metals, or an alloy of these metals is preferably used for their good optical properties and thermal conductivity.
The multilayer film can be prepared by any technique such as resistance heating, electron beam vapor deposition, sputtering, and chemical vapor deposition (CVD) and the like. Among these techniques, sputtering is often employed for high mass productivity thereof. For protecting the thin multilayer film, a resin layer is formed thereon by spin coating.
Arbitrary amorphous marks can be formed on the resulting phase-change optical medium by irradiating the medium with an optionally determined laser emitting pattern (hereinafter referred to as “strategy”). These phase-change optical media are capable of direct overwrite (hereinafter briefly referred to as “DOW”) recording in which erasing and recording procedures are performed at a time.
The term “erasing” as used herein means crystallization of an amorphous mark, and the term “recording” means conversion of a crystal into an amorphous mark.
A strategy often used is a control of three-power levels: a peak power Pp, an erasing power Pe, and a bias power Pb (Pp>Pe>Pb). A mark having an optional length is recorded by employing these parameters and various pulse widths in combination.
As a modulation system of recording and reproducing of data, eight to fourteen modulation (EFM) and EFM plus (EFM+) modulation are used in CDs and DVDs, respectively. These modulation systems are mark edge recording systems, in which control of the mark length is very important. The control of the mark length is generally evaluated based on jitter properties.
These phase-change media are widely used as rewritable DVD media such as DVD-RAM, DVD−RW, and DVD+RW. The three types of DVD media have a recording capacity of 4.7 GB but have different recording linear velocities. The DVD+RW media can be used in a constant angular velocity (CAV) system and are recordable at linear velocity ranging from 3.49 to 8.44 m/s. This means that the DVD+RW media can be recorded in a constant linear velocity system at a linear velocity of 8.44 m/s, which linear velocity is higher than those in the other systems. In general, the recording linear velocity is in proportional to the data recording velocity, and the DVD+RW media can produce data records in a shorter recording time than the other systems. To produce data records in a shorter recording time, media that are recordable at a higher linear velocity have been developed in the individual systems in recent years.
To record data at a higher linear velocity (high-velocity recording), the composition of the phase-change material in the recording layer plays an important role. Above all, it is essential to increase the recrystallization critical velocity of the phase-change material.
The “recrystallization critical velocity” as used herein is defined in the following manner.
A phase-change optical media is irradiated with DC light at a constant laser power while tracking at an optionally varying rotational linear velocity, and a change in reflectance in this procedure is determined. The laser power used herein is sufficient to fuse the phase-change material. FIG. 3 illustrates an example of the result, in which the reflectance sharply drops at a rotational linear velocity of around 5 m/s. Such a phase-change material is so designed that the reflectance in a crystalline phase is higher than that in an amorphous phase, and the phase-change material is thereby not converted into the crystalline phase, namely is not recrystallized, at a rotational linear velocity of 5 m/s or more. This critical rotational linear velocity is defined as the recrystallization critical velocity.
If the recrystallization critical velocity is lower than the recording linear velocity, the phase-change material is not sufficiently crystallized in overwriting, and the produced records are not sufficiently erased. In particular, the present inventors have confirmed by experiment that jitter significantly increases in a first overwriting (hereinafter briefly referred to as “DOW1”).
When the recrystallization critical velocity is increased, the resulting medium will have markedly deteriorated archival stability and reliability. To avoid these problems, Ge or N is incorporated into the phase-change material as disclosed in JP-A Nos. 2000-229478 and 2001-199166. However, the present inventors have verified by experiments that the recrystallization critical velocity is decreased by addition of these elements in proportional to their amounts. At some set recrystallization critical velocities, these elements may not be added in sufficient amounts to improve the archival stability and reliability.
The optical recording media should preferably be recorded at low linear velocities as well as high linear velocities, for achieving compatibility with optical medium drives already commercially available, i.e., for achieving “downward compatibility”. When an optical recording medium that can be used at a high linear velocity is used at a low linear velocity, heat generated as a result of laser irradiation readily accumulates in the recording layer. In addition, the phase-change material has a high recrystallization critical velocity. For these reasons, the phase-change material is markedly recrystallized and becomes resistant to conversion into an amorphous phase. To avoid this problem, the optical recording medium must have a “quenching” layer structure that can highly effectively dissipate heat. In addition, the minimum power Pb must have a longer pulse width and Pp must have a shorter pulse width in the strategy of laser. By employing these configurations, the generated heat can be rapidly dissipated, and the phase-change material can become amorphous. However, these configurations invite an increased recording power necessary to raise the phase-change material to a temperature at which the phase change occurs, and the resulting medium may not have satisfactory downward compatibility due to insufficient power.
In addition to the aforementioned techniques, JP-A No. 08-267926 discloses an optical disc having a specific composition of AgInSbTeGe and having high reliability in recording at high linear velocity; JP-A No. 2000-229478 discloses an optical recording medium which has a specific composition of AgInSbTeGe, is recordable with stable dimensions of fine marks of 350 nm or less and can exhibit excellent thermal stability; JP-A No. 2000-322740 discloses an optical recording medium which has a specific composition of AgInSbTeGe and is recordable and reproducible over a wide range of linear velocities; JP-A No. 2001-199166 discloses an optical disc which has a specific composition of AgInSbTeGe and has excellent overwriting properties; JP-A No. 2001-283462 discloses an optical disc which has a specific composition of AgInSbTeGe, exhibits less optical deterioration in reproduction, has good archival reliability and satisfactory sensitivity; JP-A No. 2002-103810 discloses an optical disc which has a specific composition of AgInSbTeGe, can satisfactorily overwrite at high velocity, exhibits less optical deterioration in reproduction and has satisfactory archival reliability; JP-A No. 2002-205459 discloses an optical recording medium which has a specific composition of AgInSbTeGe and is recordable and reproducible over a wide range of linear velocities.
However, these techniques do not teach improvement in overwriting properties, in particular at DOW1, and improvement in recording linear velocity and recording sensitivity. The optical recording media disclosed in JP-A Nos. 08-267926 and 2002-205459 have a recording density lower than that in the present invention, and the media disclosed in JP-A Nos. 2001-199166 and 2002-103810 can be applied only to recording at linear velocities within a narrower range than the present invention.
International Publication No. WO/97-32304 discloses an optical disc having an interfacial reflection control layer arranged above and below a recording layer and thereby having controlled optical properties for higher recording density. However, concrete materials for, and an intended purpose of, the interfacial reflection control layer are different from those in the present invention.
JP-A No. 2000-182277 discloses an optical disc having an absorption compensation layer and an interfacial layer and thereby having improved disc properties. However, concrete materials and configurations of these layers are different from those in the present invention.
JP-A Nos. 2000-348380 and 2001-006213 disclose optical discs having a transparent dielectric layer (interface layer) mainly comprising an oxide having a refractive index of 1.5 or more and zinc sulfide, and thereby having improved properties. However, concrete materials and configurations of these layers are different from those in the present invention.
JP-A No. 2002-04739 discloses an optical disc having an absorption compensation layer and an interfacial layer to thereby improve disc properties. However, concrete materials and configurations of these layers are different from those in the present invention.
JP-A No. 11-339314 discloses an optical disc having an oxide layer between a first dielectric layer (interface layer) and a recording layer to improve disc properties. However, concrete materials and thickness of the layers are different from those in the present invention.